In recent years, fiber optics have formed the basis for many different types of sensors, such as pressure sensors, strain sensors, and others. Optical fiber sensors can use phase, polarity or intensity modulation. Intensity modulation yields fiber optic sensors that are simpler and less expensive. The theory of intensity modulated optical fiber sensors, and examples of such sensors, are disclosed in the U.S. Government Patents by Nicholas Lagakos et al. Recently, intensity modulated optical fiber sensors have been adapted to measure electromagnetic phenomena, including electric and magnetic fields, voltage, and current, as disclosed in the Electromagnetic Phenomena Sensor Patent Application.
Electrical power monitoring and control systems require specialized sensor devices. The requirements associated with such sensors are determined by the particular monitoring and control approach employed, as well as the kind of performance expected of the monitoring or controlling system. A common type of electromagnetic sensor are instrument transformers, which measure current and voltage in a circuit when the electrical potential or current is too high to be applied directly to the measuring instruments. These transformers operate to reduce the current or voltage that is proportional to the current or voltage in the circuit, which can then be connected to measuring and control instruments. While electro-mechanical instrument transformers have been in use in electric power systems for many years, the accuracy of these devices is limited by a number of factors which impose significant measurement errors in field applications. In addition, these devices are often physically complex, are sensitive to electromagnetic interference (EMI), can be dangerous to humans in operation, and aren't physically robust.
A particular challenge in measuring voltage is associated with direct current electric power systems. Conventional measurement approaches, such as current transformers, and potential transformers physically require an alternating current in order to generate measurement, and therefore cannot provide measurement in direct current systems. An alternative approach, used to measure voltages in direct current systems, is to use a resistive voltage divider, which steps down voltage to a level where measurement is possible. However, these devices are resistive by their nature, which requires the use of coolants to avoid excessive heat generation, creates measurement drift over time as the resistor breaks down, and results in devices with relatively short useful lives. When these devices fail, they pose a safety hazard. These challenges, which become increasingly severe in high voltage applications, have led to the use of inferential approaches to the measurement of voltage of direct current systems operating at high voltages, such as transmission lines in electric networks, which results in an indirect measurement with an inherent rate of error.
A solution to this challenge is to employ measurement devices that use fiber optics for measurement. The use of fiber optics for sensors in general, and for electromagnetic measurements in particular, is an alternative that addresses the accuracy, EMI sensitivity, safety, size, and robustness concerns inherent in existing electro-mechanical electric system measurement. Since fiber optics use light rather than electricity as the basis for measurement, a fiber optic sensor is generally insensitive to EMI and is therefore more efficient in an environment that has a large amount of electromagnetic energy. As a result, fiber optic sensors can be located adjacent to or attached to circuits that generate large electro-magnetic fields without negative effects to either the measurement or the measuring equipment.
Intensity modulated fiber optic sensors are an alternative means of achieving accurate measurement of electric fields and voltage in both direct current and alternating current systems. It is an object of this invention to offer an fiber optic sensor that is: highly accurate in the measurement of direct current and alternating current power; linear in output measurement; capable of use in areas with high potential for EMI; physically robust; and physically simple.